1. Field of the Invention
The present invention relates to a reversible thermochromic composition. Specifically, the present invention relates to a reversible thermochromic composition which develops a clear color in red hue, especially a vivid pink color in a colored state, and becomes colorless in a decolored state.
2. Related Background Art
There have been several propositions on a reversible thermochromic composition which comprises a solubilized mixture of an electron-donating color-developing organic compound, an electron-accepting compound and a reaction medium to cause a reversible electron-exchange reaction between them in a specified temperature range, as disclosed in U.S. Pat. No. 4,028,118, U.S. Pat. No. 4,720,301, and U.S. Pat. No. 4,732,810.
For developing a red color, a benzofluorane type compound is conventionally used as the electron-donating color-developing organic compound in the reversible thermochromic compositions. However, a composition containing the benzofluorane compound develops a relatively dark bluish pink color, not suitable when a vivid pink color is required and useful only in limited application fields.
Rhodamine lactone type compounds can be also used as the electron-donating color-developing organic compounds for development of a red color. However, a composition containing the rhodamine lactone type compound, even though it develops a clear red color, leaves residual color in a decolored state, and is not suitable for practical use.